1. Field of the Invention
The present invention relates to a variable optical attenuator and specifically relates to a variable optical attenuator to be applied to a digital coherent receiver.
2. Description of the Related Art
A digital coherent transmission technology is attracting attention as a technology to realize a large-capacity optical communication of a 100 Gbps per wavelength class. This transmission technology is a technology to transmit information using both of the amplitude and the phase of the light. The digital coherent receiver is a primary constituent element of the transmission technology and is a device that converts the information of both of the amplitude and the phase of the light to an electric signal. The Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK) system is well known as a system related to the digital coherent transmission technology.
FIG. 1 depicts a configuration of a digital coherent receiver 100 as a primary constituent element of the digital coherent transmission technology. Shown here is the digital coherent receiver 100 of the DP-QPSK system. The receiver 100 has a function of performing signal demodulation by causing a light signal 140 from an information transmission path and a local light 141 to interfere with each other and is composed of a part called a light receiving front end (FE) 110 and an analog-digital converter (ADC) 120 and a digital signal processor (DSP) 130 arranged at the stage subsequent thereto.
The light receiving FE 110 is composed of a light signal processing unit 111 and an OE (Optical/Electrical) converting unit 112. The light signal processing unit 111 includes polarization beam splitters (PBSs) 113 and 114 and a 90° optical hybrid 115. The OE converting unit 112 includes a photodiode (PD) 116 that converts the light output from the optical signal processing unit 111 to the electric signal and a transimpedance amplifier 117 to perform an impedance conversion and an amplification of the electric signal, outputting it as a voltage signal. The light receiving FE 110 converts an input DP-QPSK-modulated light signal to four pairs of high-speed differential electric signals of differing light polarization components and phase components and outputs them. The digital coherent transmission technology is described in detail in “NTT Technical Journal 2011 vol. 23 No. 3, Research and Development of Optical Component Technology for 100 Gbit/s Digital Coherent Communication, ‘100 Gbit/s Light Receiving FE Module Technology’” (Nippon Telegraph and Telephone Corporation, URL: http://www.ntt.co.jp/journal/1103/files/jn201103046.pdf).
Incidentally, an optical attenuator 1 is an optical circuit component, lying in the transmission path of the optical signal 140 to be input to the light receiving FE 110 of the digital coherent receiver 100 described above, to variably control the intensity of the optical signal and is composed of, for example, a polarizer and an analyzer having a crossed Nichol relationship to each other arranged along the optical axis and a variable Faraday rotator that variably controls the rotational direction of the polarization plane of the linearly-polarized light between the polarizer and the analyzer.
The variable Faraday rotator is configured to include a Faraday element composed of magneto-optical materials and a magnetism applying means of applying a magnetic field to this Faraday element. The magnetism applying means is capable of variably controlling the direction and the magnitude of the magnetic field and is composed of a permanent magnet to magnetically saturate the Faraday element and a coil (electromagnet) to apply a variable magnetic field perpendicular to the field direction of the permanent magnet to the Faraday element.
In the variable optical attenuator having the variable Faraday rotator of the above configuration, when the light enters from the polarizer side, the linearly-polarized light transmitted by the polarizer enters the Faraday rotator and at this moment, by variably controlling the magnitude of the current applied to the coil making up the electromagnet, the polarization plane of the incoming linearly-polarized light can be rotated at an arbitrary angle. By this, the light of the intensity corresponding to the angle at which the polarization plane of the linearly-polarized light transmitted by the variable Faraday rotator and the optical axis of the analyzer cross is output from the analyzer. A configuration, an operation, etc., of the variable Faraday rotator are described in Japanese Laid-Open Patent Publication No. 1997-61770.
As described above, the digital coherent transmission technology has the digital coherent receiver as its primary constituent element. With this receiver alone, however, the optical communication using the digital coherent transmission technology is not realized. Namely, the variable optical attenuator to adjust the level of the light signal to be transmitted to the receiver becomes an essential component. Therefore, in the case of evaluating the performance, etc., of the digital coherent receiver, discussion should be made based on the configuration of the digital coherent receiver and the variable optical attenuator.
The digital coherent receiver includes the polarization beam splitter and to connect two optical devices of the digital coherent receiver including this polarization beam splitter and the variable optical attenuator, optical fibers of the variable optical attenuator and the polarization beam splitter and the optical fibers of the polarization beam splitter and the 90° optical hybrid are respectively fused to each other. For this reason, a space is required for spreading out the optical fibers to be fusion-spliced to each other. Of course, a space is also required for separately storing the variable optical attenuator and the polarization beam splitter. There are two points at which the optical fibers are fused to each other and deterioration and loss of the optical signal are feared.
Surely, the 90° optical hybrid and the polarization beam splitter making up the digital coherent receiver can be integrated as one unit but the 90° optical hybrid is substantially a Mach-Zehnder interferometer formed by a planar lightwave circuit (PLC) and the PLC is not suitable for miniaturization. For this reason, when the 90° optical hybrid and the polarization beam splitter are integrated as one unit, not only the Mach-Zehnder interferometer but also the polarization beam splitter is formed by the PLC and an increased size is unavoidable. Anyway, there has been a problem that it is extremely difficult to achieve the configuration containing the digital coherent receiver and the variable optical attenuator and the miniaturization of the digital coherent receiver itself.